The present invention relates to a liquid crystal display having a network structure, the display being made of a cholesteric liquid crystal layer, a chiral nematic liquid crystal layer, or a liquid crystal layer, being a combination thereof.
As shown in FIG. 5, a planar array structured liquid crystal display is a liquid crystal display having chiral nematic liquid crystal 1 having a cholesteric layer retained between a pair of ITO 2a and 2b and substrates 3a and 3b, in which the helical axes of liquid crystal particles are regularly disposed perpendicular to the plane of the substrates 2a and 2b. The liquid crystal display meets the Bragg""s reflection conditions, in which light entering from one substrate 3a side is reflected onto the rear side (substrate 3a side). At this time, it has been publicly known that if the helical axes of the planar array are excessively aligned, the visual angle dependency (a characteristic by which wavelengths of the Bragg""s reflection light change according to the angle of looking at the reflection light) is increased.
Also, if the perpendicular array property with respect to the planes of the substrates 3a and 3b of the helical axes of the planar array is disordered, and becomes a focal conic array in which the helical axes of the liquid crystal 1 are arrayed in an irregular direction or almost parallel to the surface of the substrates 3a and 3b, light entering from one substrate 3a side is reflected frontward, passing through the other substrate 3b, after it is subjected to Bragg""s reflection on the helical plane of liquid crystal. In addition, in the respective liquid crystal in which the perpendicular array property with the substrate plane of the helical axes is disordered, the angles that causes the incident light to be subjected to Bragg""s reflection by the liquid crystal array differ, wherein the wavelength dependency of the reflective index is eliminated, and the reflected light seems to be whitened, although depending on the positions of observation of the reflected light.
In PDLC (Polymer Dispersed Liquid Crystal) or PNLCD (Polymer Network Liquid Crystal Display) which is a liquid crystal display in which normal nematic liquid crystal is capsulated and dispersed in a macromolecular resin, an optical scattering mechanism that is brought about by mismatching of the refractive indexes between the liquid crystal and macromolecular resin. Therefore, almost all light entering from one substrate 3a side is scattered in the liquid crystal layer, and thereafter it passes through the other substrate 3b and is moved frontward. Therefore, it is difficult to achieve a reflection display having a sufficient brightness.
Contrary to this, since, in the cholesteric liquid crystal or chiral nematic liquid crystal, light entering from one substrate 3a side is subjected to reflection, the quantity of light reflected onto the rear side of the corresponding one substrate 3a side becomes sufficient.
However, since the visual angle dependency is high, resulting from the Bragg""s reflection, chromatic changes or glare occurs by changing the visual angle. To eliminate these faults, as shown in FIG. 6, a slight amount of monomer is doped to liquid crystal and polymerized to make a polymer network 4 for the purpose of scattering the perpendicular array property of liquid crystal 1 with respect to the planes of a pair of ITO 2a and 2b and substrates 3a and 3b of the helical (spiral) axes. This has been reported in the literatures ((1) Tokuhyo H7-507083, (2) xe2x80x9cMaterials for Polymer-Stabilized LiquidCrystalxe2x80x9d L.-C. Chien, etc., Kent State University, 1996, Chapter 11, P.182-P.189, American Chemical Society), etc.
Since very minute polymer networks 4 are three-dimensionally formed of a monomer such as BMBB6 (4,4xe2x80x2-bis{4-[6-(methacryloyloxy)-hexyloxy]benzoate}-1,1xe2x80x2-biphenylene), etc., proposed by Kent, et al., the whiteness of the reflection light is increased, and glare may be eliminated. But, sufficient reflection light can not be obtained.
Also, although not being limited to the invention made by Kent et al., in a liquid crystal display of a network structure in which liquid crystal is dispersed in macromolecular resin produced by prior arts, since the size of the network is small, sufficient reflection can not be obtained from the planar texture, wherein light is scattered frontward (the substrate 3b side) due to the polymer network which becomes the domain boundary, and the reflective index may be worsened.
Also, as a liquid crystal display obtained by the method of dispersing a small amount of macromolecular resin in the chiral nematic liquid crystal, PSCT (Polymer Stabilized Cholesteric Texture) has been already known, which becomes a transparent body of homeotropic structure when voltage is applied, and a reflection body of planar structure or a scattered transparent body of slight focal conic structure due to a switch-off wavelength of voltage when no voltage is applied. The quantity of monomer doped on the PSCT is approx. 3%, which forms minute networks of a size of 1 micron. Since the size of the networks is very minute, the shock resistant property is inferior, wherein the networks may be easily deformed due to external forces to cause the internal liquid crystal texture to be deformed. That is, such a problem arises, where if the surface of the liquid crystal panel is pressed by a finger, the display may disappear.
Although a liquid crystal display provided with a touch panel is available, the abovementioned PSCT cannot be used for this type of use. In order to improve the shock resistant property of the PSCT, it is publicly known that doping of a greater quantity of monomer is effective. But, if 10% or more monomer is doped in, for example, the PSCT made by Kent, et al., cases where light is scattered in the polymer networks are increased, the reflective index is comparatively decreased, and light scattering at the focal conic portion is increased, whereby the contrast of the liquid crystal display is worsened.
Also, although the monomer doped in liquid crystal is polymerized by radiation rays such as ultraviolet rays or heat, and polymer networks are formed, the surface area which is brought into contact with the liquid crystal is increased because the polymer networks are very minute, whereby non-reacted monomer contained in the polymer is easily eluted into the liquid crystal.
It is therefore an object of the invention to provide a liquid crystal display which has excellent reliability in terms of optical characteristics and shock-resistance properties.